91 J RRC 3.9 Autp Not Shifting into top gear

did whaddon and abused it drive home and it was fine, drove this morning on the motorway and its not shifting into top gear. tried shifting through 1 2 3 D but that didnt help either, the exhuast is blowing badly at the manifold or down pipe quite badly now, are there ay quick things to check here? is there a kick down cable/wire, vaccum etc etc that i can check out? otherwise around town she drives sweet as, its just evident on the motorway

any ideas please/??????